Fungibility
Fungibility is a property of money and commodities. It means that each unit can be exchanged for each other unit. Fungibility is very important for Bitcoin. Every satoshi should be equivalent to every other satoshi.
Using techniques such as CoinJoin and CoinSwap can really help with long term fungibility of bitcoin.
In 2018 OFAC added 2 bitcoin addresses on a sanction list. If this practice is continued and enforced it could really harm fungibility.